1. Field of the Invention
This invention relates in general to seat cushions and more specifically to an adjustable seat cushion having tension limiting means for protecting an occupant in the event of vertical vehicular crashes, rocket-assisted ejection from an aircraft and the like.
2. Description of the Prior Art
As is well known in the art, the human body has a relatively low tolerance to compressive loads in the spine. Because of this, special seating systems have been devised and are required in some instances where compressive loading of the spine can be expected, such as in crashworthy helicopter seats, ejection seats of fighter aircraft, and the like.
Two basic factors must be considered when designing such a special seating system, and those two factors are safety and comfort. All seats used in the cases discussed above, i.e. helicopter seats, aircraft ejection seats, and the like, include among other things, a rigid seat pan. For comfort considerations an occupant cannot sit directly on the seat pan for very long. Thus, it is a common prior art practice to interpose some sort of a cushion between the seat pan and the occupant's buttocks.
Whenever the occupant's buttocks is displaced relative to the seat pan, such as by a cushioning structure, the effects of vertical acceleration and/or deceleration come into play. More specifically, in the case of vertical acceleration, such as in an ejection seat, the seat pan will accelerate more rapidly than the occupant and can reach spine damaging velocities by the time it impacts the occupant's buttocks if displacement of the occupant's buttocks relative to the seat pan is excessive prior to the start of acceleration. Similarly, the occupant can crash down onto the seat pan which has been stopped as a result of abrupt vertical deceleration, such as in a helicopter crash.
Several prior art structures have been devised as compromised attempts to satisfy the mutually exclusive factors of safety and comfort which must be considered in seats of this type.
A first prior art attempt was to use a very thin or very compressible seat cushion which was very close to being completely flattened, or crushed, by the weight of the occupant under the influence of normal gravity. This was less than satisfactory in that the cushion had to be designed for an average occupant and thus could not take into account a wide range of occupant body variables such as weight, bone structure, buttock flesh, posture and the like. Even with an average occupant, comfort was lacking in all but very short sitting periods.
The use of the very thin or very compressible cushion was improved to a degree by using it in conjunction with a seat pan that is formed with a buttock contour. This second prior art seating system, which is currently used in many safety seating systems, distributes the load more uniformly but still suffers from the same drawbacks as the first prior art system in that both the cushion and the contoured seat pan must be designed for an average occupant.
Another prior art attempt at solving the safety seat problem is to make a cushion out of a material which may be referred to as a rate-sensitive material, that is, one that behaves rigidly during rapid acceleration and deceleration, and is readily deformed in normal use. Examples of such materials which have been tried with various degrees of success, are rate-sensitive upholstery foams whose stiffness depends on the rate of compression, viscous fluids in bladders, viscous solid elastomeric materials, beads in bags and non-Newtonian fluids whose stiffness increases dramatically with the shear rate. Seat cushions formed of some of the rate-sensitive materials mentioned above have been judged by some users to be comfortable, but only the non-Newtonial fluid has demonstrated truly rigid behavior in resisting abrupt displacement. Dynamic test data on rate-sensitive materials per se has typically been obtained under nonrepresentative conditions and because of this, there is reason to suspect the use of such materials in safety seating systems. In addition, cushions made of rate-sensitive materials tend to be heavy and lack ventilation capabilities and thus require the use of an additional ventilation cushion or layer.
Still another prior art attempt has been made to solve this safety seat problem by using what is referred to as a diaphragm-type seat cushion. A diaphragm-type cushion is one which supports the occupant on a stretched membrane of net material, fabric or an elastomeric sheet. This approach has not proven to be satisfactory for several reasons. If the diaphragm is formed of a stiff material having little or no buttock conforming capability, then it can be suspended reasonably close to the seat pan for safety reasons. However, the stiff material will be about as comfortable as the thin and/or compressible cushion pad used on the contoured seat pan. On the other hand, if the diaphragm is formed of a flexible material having buttock conforming capabilities, it must be suspended quite high above the seat pan to provide sufficient room for the membrane to conform to the occupant's buttocks. The problem with this latter approach is that it is virtually impossible to determine a diaphragm suspension point which will be ideally suited for all occupant variables. That is, some occupants will displace the diaphragm more than others with the result being that some occupants will be sitting directly on the uncomfortable seat pan, while others will be suspended above the seat pan a distance which could damage their spines in the event of rapid vertical acceleration or deceleration.
Therefore, a need exists for a new improved seat cushion which overcomes some of the problems and shortcomings of the prior art.